The present invention relates to a vacuum brake booster and more particularly to a reaction lever mechanism therefore.
From U.S. Pat. No. 3,102,453, a vacuum brake booster is known which comprises a compartment in which a constant vacuum prevails, a compartment in which different pressures prevail, a movable wall dividing the compartments and being mounted on a plunger or push rod acting on a master cylinder piston, and a control valve carried by the movable wall which is actuatable by a brake pedal. The control valve controls the differentials of pressure acting on the movable wall. The end of the plunger close to the control valve is provided with a reduced-diameter end onto which a reaction disc or plate is pressed. Between the reaction disc and the control valve, three reaction levers are positioned which on one side bear against the movable wall radially outwardly and against a reaction-delaying spring radially inwardly, and on the other side bear against the disc. In this arrangement, the reaction-delaying spring is positioned in an opening, close to the vacuum compartment, in the valve piston of the control valve. The biasing force of the spring provides for what is termed a "two-stage reaction" which means that there is a retardation of the reaction force acting on the brake pedal.
The magnitude of the two-stage reaction is dependent upon the strength of the reaction-delaying spring and the distances between the points of contact of the reaction levers. Since the space available for accommodating the spring is limited, its effect can only be changed by changing the leverage. A displacement of the points of contact where the reaction levers are in engagement with the spring and the movable wall entails substantial difficulties because of the resulting major constructional changes.
The reaction disc or plate positioned at the end of the push rod and engaging the reaction levers is subjected to high bending stresses. In plates including a bore, the permissible bending moment is substantially lower than in arrangements where the reaction plate is butt welded to an end of the push rod. The permissible bending moment is particularly low in reaction plates having a central bore for fastening to the push rod and of rectangular construction because only two reaction levers are provided. The permissible bending moment may be increased by providing thicker reaction plates which are, however, heavier and more expensive.
As mentioned previously, arrangements are known in which the reaction plate is butt welded to an end of the push rod. In such arrangements, the geometrical moment of inertia of the reaction plate is high because a bore is not necessary. When welding the parts together, extreme care must be taken to achieve precise centering.
To achieve proper functioning of the reaction lever mechanism and a precise two-stage reaction operation in the prior known brake boosters, it is necessary to provide for an accurate support for the push rod at the end adjacent the control valve in order for the reaction plate edge to act upon the reaction levers at the desired point.
In the known brake boosters, different two-stage reactions may be produced by mounting push rods with reaction plates of different size. Since, however, the brake boosters are equipped with different push rods and different types of reaction plates, there results a considerable number of combination possibilities. For each booster type, a complete pre-fabricated push-rod-and-reaction-plate unit must be made available.